1. Field of the Invention
This invention relates to electronic equipment and, in particular, to printed circuit boards.
2. Art Background
Printed circuit boards are widely used in electronic equipment. In such boards generally a metallic, e.g., copper, pattern is produced on a polymer substrate. The substrates are typically fabricated by spraying a lacquer including the desired resin, for example, an epoxy resin, onto a reinforcing grid such as a fiber glass mesh. A variety of methods are employed to produce the copper pattern. These methods typically involve a lithographic process and a wet etching process which, although quite reliable, sometimes yield irregularities in the desired pattern. For example, discontinuities or width narrowing or broadening in metallic pattern lines are encountered. Such irregularities often lead to the degradation of component properties and ultimately to equipment failure.
It is desirable to detect irregularities before further processing occurs. After the metal pattern is produced, electronic components are affixed to the pattern through various expedients such as soldering. Obviously, if the pattern produces a failure after the components are affixed, a large expense is incurred in either replacing the board or, alternatively, in discovering the point of failure and repairing it. In contrast, if the irregularity is detected before affixing of the electronic components, the relatively inexpensive board is either discarded or expeditiously repaired.
Various ways have been investigated to determine the presence of pattern flaws before installation of circuit components. For example, as disclosed in U.S. Pat. No. 4,152,723, filed May 1, 1979, (which is hereby incorporated by reference) pattern flaws are detectable using a luminescence technique. In particular, a light source such as a laser is employed to induce a fluorescence in the polymer substrate. Only the portions of the substrate which are not covered by the metallic pattern are illuminated and thus fluoresce. The ambits of the metallic pattern are to an extent determined by sweeping a light beam across the patterned surface and monitoring with conventional equipment, such as a photomultiplier, the fluorescence intensity fluctuations during the sweep. However, this technique does not lead to the reliable observation of pattern flaws.